1. Field of the Invention
The present invention relates to a two-photon absorption decolorizable material characterized in having a large non-resonant two-photon absorbing cross section and capable of decolorizing the non-resonant two-photon absorption dye itself or a separately added decolorizable dye, and a two-photon absorption material for refractive index modulation or absorbance modulation utilizing the same, and a three dimensional optical recording medium.
2. Description of the Related Art
A non-linear optical effect generally indicates a non-linear optical response proportional to a second, third or higher power of an applied photoelectric field, and a second-order non-linear optical effect, proportional to a square of an applied photoelectric field includes a second harmonic wave generation (SHG), a photorectification, a photorefractive effect, a Pockels effect, a parametric amplification, a parametric oscillation, a photofrequency additive mixing and a photofrequency subtractive mixing. Also a third-order non-linear optical effect, proportional to a cube of an applied photoelectric field includes a third harmonic wave generation (THG), an optical Kerr effect, a self-induced change in refractive index, and a two-photon absorption.
A non-linear optical material showing such non-linear optical effect, various inorganic materials have been found. In such inorganic materials, however, a practical application is very difficult because of a difficulty in so-called molecular designing for optimizing the desired non-linear optical characteristics and physical properties required for the preparation of a device. On the other hand, organic compounds have a higher possibility of practical application as they can be optimized, by a molecular designing, to desired non-linear optical characteristics and can be controlled in various physical properties, and are attracting attention as a promising non-linear optical material.
Among the non-linear optical characteristics of the organic compounds, a third-order non-linear optical effect is recently attracting attention, and a non-resonant two-photon absorption is attracting a particular attention. A two-photon absorption means a phenomenon of an excitation of a compound by simultaneously absorbing two photons, and a case where the two-photon absorption takes place in an energy region in which the compound does not have a (linear) absorption band is called a non-resonant two-photon absorption. In the following description, a two-photon absorption means a non-resonant two-photon absorption unless specified otherwise.
An efficiency of the non-resonant two-photon absorption is proportional to a square of a photoelectric field (square characteristics of two-photon absorption). Therefore, in case of a laser irradiation on a two-dimensional plane, the two-photon absorption takes place only in a position of a high electric field strength at a center of the laser spot, and does not take place at all in a peripheral portion where the electric field strength is lower. On the other hand, in a three-dimensional space, the two-photon absorption takes place only in a region of a high electric field strength at a focal point where the laser beam is condensed by a lens, and does not take place at all in a region outside the focal point because of a lower electric field strength. In contrast to a linear absorption which causes an excitation in all the positions in proportion to the strength of the applied photoelectric field, the non-resonant two-photon absorption causes an excitation only in a point within the space, based on the square characteristics, and can therefore significantly improve the spatial resolution.
In case of inducing a non-resonant two-photon absorption, there is generally employed a short pulsed laser of a near infrared region which is longer in wavelength than the wavelength region of the (linear) absorption band of the compound and in which the compound does not have an absorption. Such near infrared exciting light of so-called transparent region can reach the interior of a sample without being absorbed or scattered, and can excite a point in the interior of the sample with an extremely high spatial resolution, owing to the square characteristics of the non-resonant two-photon absorption.
On the other hand, an optical information recording medium (optical disk) capable recording information only once with a laser light is already known, and an add-on type CD (so-called CD-R), an add-on type DVD (so-called DVD-R) and the like are already commercialized.
A DVD-R is representatively constituted, on a transparent disk-shaped substrate having a guiding groove (pregroove) for tracking of an irradiating laser beam with a pitch of 0.74-0.8 μm less than half of that in a CD-R, of a recording layer of a dye, also a reflective layer normally formed on the recording layer, and a protective layer if necessary.
An information recording on the DVD-R is executed by an irradiation of a visible laser light (normally within a range of 630-680 nm) whereby an irradiated portion of the recording layer absorbs such light to cause a local temperature elevation, thereby causing a physical or chemical change (such as a pit formation) thereby changing the optical characteristics. On the other hand, an information reading (reproduction) is also executed by an irradiation with a laser light of a wavelength same as that of the recording laser light, and the information is reproduced by detecting a difference in the reflectance between a portion of the recording layer where the optical characteristics are changed (recorded portion) and a portion where the optical characteristics are not changed (unrecorded portion). Such difference in reflectance is based on so-called “refractive index modulation”, and a larger difference in the reflectance between the recorded portion and the unrecorded portion favorably results in a larger ratio of the optical reflectances, namely a higher S/N ratio of reproduction.
On the other hand, the recent progress of so-called information society rapidly promotes pervasiveness of networks such as Internet and the high-vision TV. Also the broadcasting of the HDTV (high definition television) is planned shortly, and there is anticipated an increasing demand, also in consumer applications, for a high-density recording medium capable of recording image information of 50 GB or more, preferably 100 GB or more inexpensively and easily.
Also in business applications such as computer backup or broadcasting backup, a demand for an ultra high-density recording medium capable of inexpensively recording information of a large capacity of 1 TB or more, at a high speed.
However, in an existing two-dimensional optical recording medium such as DVD-R, a storage capacity is limited to about 25 GB at maximum because of the physical principle even if a short wavelength is employed for recording and reproduction, and a sufficiently large recording capacity capable of meeting the future demand cannot be anticipated.
Therefore, as an ultimate high-density recording medium, a three-dimensional optical recording medium is recently attracting attention. The three-dimensional optical recording medium is to achieve an ultra high density and an ultra high capacity of tens or hundreds of times of those of the prior two-dimensional recording medium by superposing tens or hundreds of recording layers in a third dimensional direction (direction of thickness). For realizing a three-dimensional optical recording medium, there is required an access and a writing in an arbitrary position in the third dimensional direction (direction of thickness), and candidates therefor include a method utilizing a two-photon absorption material and a method utilizing holography (interference).
A three-dimensional optical recording medium utilizing a two-photon absorption material is capable of so-called bit recording of tens or hundreds of times by the above-described physical principle and can achieve a recording of a higher density, and can therefore called an optical recording medium of an ultimate high density and an ultimate high capacity.
For a three-dimensional optical recording medium utilizing a two-photon absorption material, there are proposed, for example, a method of utilizing a fluorescent material for recording and reproduction and executing a reading by a fluorescence (Levich, Eugene, Polis et al., JP-T No. 2001-524245, Pavel, Eugene et al., JP-T No. 2000-512061), and a method of utilizing a photochromic compound and executing a reading by an absorption or a fluorescence (Corotiv, Nicolai-Ai et al., JP-T No. 2001-522119, Arsenov, Vradimir et al., JP-T No. 2001-508221), but these literatures do not disclose a specific two-photon absorption material, also only utilize, in the abstractively shown examples of two-photon absorption compound, two-photon absorption compounds of an extremely low two-photon absorption efficiency, and are unsatisfactory in a non-destructive readout, a prolonged storability of the record and an S/N ratio in the reproduction, so that these methods cannot be considered a practically acceptable system.
Also for achieving a non-destructive readout and a prolonged storability of the recording, it is desirable to employ an irreversible material and to execute a reproduction by a change in the reflectance (refractive index), but a two-photon absorption material having such functions has not been disclosed.
Also JP-A No. 6-28672 (Satoshi Kawata and Yoshimasa Kawada) and JP-A No. 6-118306 (Satoshi Kawata and Yoshimasa Kawada) disclose a recording apparatus, a reproducing apparatus and a readout method utilizing a three-dimensional recording by a refractive index modulation, but a method utilizing a two-photon absorption decolorizable material is not described.